This invention relates to sheet feeding apparatus for printing presses such as offset or letter presses, and more particularly to a dual-stream envelope feeding printing press attachment.
Sheet feeding and registration devices for printing envelopes typically include means for separating one sheet at a time from a stack of sheets, the sheets being fed serially to a conveyor for longitudinal and lateral registration, the sheets being subsequently gripped and drawn through a printing station of the press.
When printing narrow stock such as envelopes, it is advantageous to feed and print two at a time for effective utilization of the press. This is because the cost of operating the press is directly related to the number of printing cycles performed, only incidentally influenced by the total width of stock printed in a cycle.
In U.S. Pat. No. 3,893,664, a feeding attachment for use on a Multilith press is described. Envelopes are separated from the bottom of a stack and transported by vacuum suckers to a feed roller for delivery to a conveyor on the press. A pair of adjustable side guides and an adjustable back guide hold the stack in position against a stationary stop plate. A jogger paddle at one side of the conveyor moves the envelopes into lateral registration against an opposite stationary paddle. In a commercial version of the attachment, available from Press Specialties Mfg. Co., Portland, Oreg., a "2-up" accessory can be added for feeding two streams of envelopes. The accessory includes a double guide for use between the two stacks of envelopes, an additional back guide for use behind one of the stacks, a pair of secondary joggers, and a pair of secondary sheet riders. The secondary joggers are mounted, one to a stationary shaft, the other to a jogging shaft between a centermost pair of four conveyor tapes in the conveyor section of the Multilith press. Two standard sheet riders, and the secondary sheet riders are positioned, each over one of the four conveyor tapes for urging each envelope into contact with two of the conveyor tapes. As the envelopes move along the conveyor, the envelopes of each pair are jogged simultaneously in the same direction by the jogger paddles on the jogging shaft toward the corresponding stationary jogger paddles on the stationary shaft. Each pair of envelopes is subsequently printed as described above.
A disadvantage of this prior art attachment for dual-stream feeding is that it is difficult to set up. Installation and adjustment of the individual secondary joggers and secondary sheet riders is difficult, time consuming, and therefore expensive. Removal of at least the secondary joggers is also required prior to resumption of single-stream operation.
Another disadvantage of this prior art attachment for dual-stream feeding is that proper registration with the printing station is difficult to obtain. Two separate lateral adjustments are required for each stream on the conveyor, the streams required to be spaced apart a fixed distance as determined by image spacing at the printing station.
Another disadvantage of this prior art attachment for dual-stream feeding is that the secondary jogger paddles use up an excessive amount of space in the center of the press. Clearance must be provided between the laterally moving paddle mounted to the jogging shaft and another paddle on the stationary shaft. This wasted space results in dual-stream operation being limited to narrower envelopes than would otherwise be possible.
A further disadvantage of this prior art attachment is that envelopes tend to become caught between stack guides on the attachment when envelopes near the bottom of the stack have air squeezed therefrom by the weight of envelopes higher in the stack.
In U.S. Pat. No. 4,228,994, a variable jogger for lateral registration of fed sheets with respect to the center line of a printing press is described. Joggers at each side of a conveyor section of the press simultaneously move fed sheets toward the center line. Separate lateral adjustments of each jogger can be made while the press is operating. A disengagement mechanism permits separate use of either paddle. Commercial versions of the press, produced by the patent assignee, are commonly known as the Chief 15, 17, 215, and 217 Duplicators. The conveyor section of the Chief press is equipped with three conveyor tapes. Two of the conveyor tapes, and corresponding sheet riders, are adjustably positioned near opposite sides of the path of the fed sheets. The third conveyor tape is located midway between the others by a scissors mechanism coupled to the corresponding outside conveyor tape adjustment means. The patent describes dual-stream feeding with the joggers contacting one side only of the fed sheets. A version of the prior art feeding attachment described above can be used on the Chief press; however, successful dual-stream envelope feeding thereon has heretofore not been achieved. It is believed that one or more of the following problems have contributed to the lack of successful dual-stream envelope feeding on the Chief press:
1. Three conveyor tapes. For proper angular alignment of the fed sheets on the conveyor section, each sheet should be supported near opposite sides thereof by conveyor tapes and held in balanced contact therewith by corresponding sheet riders. A sheet rider like those associated with the outside conveyor tapes could be positioned over the middle conveyor tape; however, there is room for only one such sheet rider, contacting only one stream of the sheets. Therefore, unbalanced conveyor contact and angular misalignment of the sheets results from dual stream feeding over only the three conveyor tapes.
2. Single edge registration. In dual stream feeding, no stationary jogger paddle is provided on the Chief press. Consequently, envelopes tend to twist laterally out of position when jogged at one edge only, because irregular drag is produced by contact between the sheet riders and folds of the envelope.
3. Scissors mechanism. Lateral adjustment of an outside conveyor tape produces a corresponding centering lateral movement of the middle conveyor tape. Consequently, there is an interaction in the relationship of the conveyor tapes to the envelopes fed on opposite sides of the press. Additionally, the center conveyor tape cannot be positioned for equal contact with unequally sized envelopes fed on opposite sides of the press.
Accordingly, there is a requirement for a dual-stream envelope feeding printing press attachment that is easy to set up and adjust, is easy to restore to single-stream operation, provides a fixed registration distance between corresponding pairs of envelopes, does not jam when feeding envelopes, and is economical to produce.